1. Field of the Invention
The present invention relates to an impressed current cathodic protection system for a marine engine. Preferably, the protection system provides a protective current flow through a coolant passage.
2. Description of the Related Art
A conventional outboard motor engine often uses seawater as a coolant and may be subject to cathodic corrosion due to the seawater contacting the inner wall of its coolant passages. In order to inhibit cathodic corrosion, an anticorrosive coating commonly is applied to an inner surface of at least some of the coolant passages within the engine. The anticorrosive coating may be applied by painting the coating on an inner wall of the coolant passage. The anticorrosive coating applied within the coolant passages may come off after the engine has been in service for a long period of time. Once the coating comes off, the coating no longer inhibits corrosion.
In some outboard motors, a cathodic protection system is used to inhibit electrolytic corrosion. For example, a corrosion protection system may use a sacrificial electrode or anode in the shape of a bar. The anode may be detachable from a cylinder head with its electrode facing an internal space of the coolant passage.
At least two types of sacrificial anodes often are used for conventional outboard motor engines. The first type of anode is externally attached to/detached from the engine while the other type of anode is internally attached to/detached from the engine. Self-corrosion of the anode produces a protective current which causes the anode to be consumed. The consumption of the anode creates a need for replacement before the anode is completely consumed.
The residual current of the anode is measured periodically or at the time of engine maintenance and, if necessary, the anode is replaced. The anode can also be visually inspected to determine whether replacement is required. To visually inspect the residual anode, the anode is removed from the engine or the engine is disassembled.
A disadvantage of this system is that part of the internal space of the coolant passage is used for setting the anodes. For anodes designed to be externally attached to/detached from the engine, a dedicated mounting seat on the external surface of the engine also must be provided. In addition, each anode is only effective over a limited area, and thus multiple anodes may be required for complete protection. An engine equipped with a cathodic prevention system that uses the aforementioned anodes tends to be larger. The extra assembly step of attaching the anode also increases manufacturing costs. Further, the measurement of the residual anode current and the replacement of the anodes increase maintenance costs.
Known impressed cathodic protection systems utilize an anticorrosive electrode in the coolant passage and more specifically on the upstream side of the engine. See, e.g. Japanese Publication No. 06-299377, dated Oct. 25, 1994. The anticorrosive electrode provides an anticorrosive effect to an area adjacent to the anticorrosive electrode; the anticorrosive effect is limited for other areas of the coolant passages through the engine, especially at the more narrow passages.